website: Add documentation for gem5art Change-Id: Icc61f3a1304bb61575bb5adbb704c58a6b0f689f Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/43685 Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu> Maintainer: Jason Lowe-Power <power.jg@gmail.com> Tested-by: Jason Lowe-Power <power.jg@gmail.com>